1. Field
The present disclosure relates generally to communication systems, and more particularly, to a hybrid R-2R structure for low glitch noise segmented digital-to-analog converter (DAC).
2. Background
A wireless device (e.g., a cellular phone or a smartphone) may transmit and receive data for two-way communication with a wireless communication system. The wireless device may include a transmitter for data transmission and a receiver for data reception. For data transmission, the transmitter may modulate a transmit local oscillator (LO) signal with data to obtain a modulated radio frequency (RF) signal, amplify the modulated RF signal to obtain an output RF signal having the desired output power level, and transmit the output RF signal via an antenna to a base station. Additionally, the transmitter may include a DAC to assist in the generation of the output RF signal that is transmitted.
A DAC converts digital signals (e.g., a 4-bit DAC converts a digital word of four bits, such as a digital signal of 0110) into a corresponding current or a corresponding analog voltage. A four-bit DAC will produce a different analog voltage value, or a different amount of current, for each possible digital value. That is, the four-bit DAC will produce a different current or analog voltage for each value of the digital signal from 0000 to 1111.
Low-noise, low-power, wideband, and high-resolution DACs are increasingly sought for advanced wireless standards, such as long term evolution (LTE), in which the DAC is modified for higher data rates or bandwidth. Such DACs may typically limit architecture and design choices.
For a transmit (TX) DAC used in RF applications, minimizing high frequency glitch noise, also referred to as DAC noise, at certain offset frequencies may avoid certain operational issues in a broadband cellular TX path. That is, it may be useful to reduce or to cancel such glitch noise, because the noise may desensitize an associated receive (RX) channel. In other words, out-of-band noise from the TX DAC can fall into the RX-band, and thereby desensitize the RX channel. Such out-of-band noise may be an issue of particular concern for the surface-acoustic-wave filterless LTE (SAW-less LTE) system.
Attempts to reduce glitch noise may address glitch-noise mismatch between most significant bits (MSBs) of a DAC and least significant bits (LSBs) of the DAC, may require careful scaling, or may take into consideration robustness to process, voltage, and temperature (PVT) variations. For example, an approach to optimize delay between MSBs and LSBs may result in relatively high PVT variations, wherein the DAC may produce different analog voltages/currents for an identical digital signal depending on process variation, how long the DAC has been operating, or depending on a temperature of the DAC.
Another approach may be to increase the resolution of the DAC to lower the entire quantization noise floor. However, such an implementation may be expensive, due to significant resultant increases in area, power, or voltage headroom.